Microtearing Turbulence and Its Role in High-Density-Gradient Plasmas in Wendelstein 7-X

Abstract

Gyrokinetic simulations reveal that microtearing mode (MTM) turbulence dominates transport in a Wendelstein 7-X (W7-X) discharge characterized by large density gradients, moderate temperature gradients, and low plasma beta. This conclusion is supported by the close agreement between simulated and experimentally measured heat and particle fluxes. The emergence of MTMs is attributed to the absence of competing instabilities -- such as ion temperature gradient modes and density-gradient-driven trapped-electron modes -- under these plasma conditions, together with the stabilizing influence of the W7-X max-J magnetic configuration. Furthermore, moderate collisionality and low magnetic shear are found to facilitate MTM onset. These findings advance the understanding of high-density-gradient regimes, which are essential for achieving high-performance operation in W7-X plasmas.

Figures (4)

• Figure 1: Linear gyrokinetic characterization of instabilities: a) Growth rate and real frequency as a function of $k_y \rho_s$. Impurity contribution (orange) is negligible. The dotted line indicates the electron diamagnetic frequency $\omega^*_{p_e}$. b) Growth rate versus normalized collision frequency for various $k_y \rho_s$. The experimental collisionality is marked by a vertical dashed line. Full circles denote MTM; hollow triangles denote a TPM. The changes for $k_y\rho_s=0.1$ were: $(n_x,n_z)=(64,288)$ and 3 npol. c) Ratio of the contributions to the growth rate from ions and electrons. d) Tearing parity at $k_y \rho_s = 0.5$: odd parity in $\phi$ (blue) and even in $A_\parallel$ (orange), normalized to their respective maxima. Only the central ballooning angle region is shown.
• Figure 2: Dependence of MTM growth rate on: a) electron temperature gradient, b) global magnetic shear, c) ion temperature gradient, and d) density gradient. The nominal NBI collisionality (solid line) and the reduced collisionality of the ECRH reference case mentioned in the text were used (dotted line). Vertical dashed lines indicate experimental values.
• Figure 3: a) Growth rates and unstable modes as a function of $k_y \rho_s$ at different radial positions. MTMs are indicated by circles and solid lines, while ITGs by hollow squares and dotted lines. b) Normalized gradient scale lengths and magnetic shear $\hat{s}$ versus radius. The color of the vertical dotted lines indicate the radial positions of the linear simulations in a).
• Figure 4: a) Time trace of flux surface-averaged electron heat and particle flux in gyro-Bohm units ($[\Gamma]=n_ec_s(\rho_s/a)^2$ and $[Q]=n_eT_ec_s(\rho_s/a)^2$). Particle flux, electrostatic and electromagnetic heat flux after increasing $a/L_n$ by 20% are shown in gray, dark green and dark blue, respectively. b) Frequency spectrum of electrostatic potential as a function of wavenumber, with the dashed line indicating the real frequency obtained in linear simulations (Fig. \ref{['F1_lin_characterization']}a). c) and d) heat and particle flux spectra of electrons and ions, respectively.